Magnetic powder, manufacturing method of magnetic powder and bonded magnets

ABSTRACT

Disclosed herein is a magnetic powder which can provide magnets having excellent magnetic properties and having excellent reliability especially excellent heat stability. The magnetic powder is composed of an alloy composition represented by R x (Fe 1-a CO a ) 100-x-y-z B y M z (where R is at least one kind of rare-earth element excepting Dy, M is at least one kind of element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at %, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and the magnetic powder being constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, wherein when the magnetic powder is mixed with a binding resin and then the mixture is subjected to injection molding or extrusion molding to form a bonded magnet having a density ρ[Mg/m 3 ], the maximum magnetic energy product (BH) max [kJ/m 3 ] of the bonded magnet at a room temperature satisfies the relationship represented by the formula (BH) max /ρ 2 [×10 −9  J·m 3 /g 2 ]≧2.10, and the intrinsic coercive force H CJ  of the bonded magnet at a room temperature is in the range of 400-760 kA/m.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to magnetic powder, a manufacturingmethod of magnetic powder and bonded magnets. More particularly, thepresent invention relates to magnetic powder, a manufacturing method ofthe magnetic powder and a bonded magnet which is manufactured, forexample, using the magnetic powder.

[0003] 2. Description of the Prior Art

[0004] For reduction in size of motors, it is desirable that a magnethas a high magnetic flux density (with the actual permeance) when it isused in the motor. Factors for determining the magnetic flux density ofa bonded magnet include magnetization of the magnetic powder and thecontent of the magnetic powder to be contained in the bonded magnet.Accordingly, when the magnetization of the magnetic powder itself is notsufficiently high, a desired magnetic flux density cannot be obtainedunless the content of the magnetic powder in the bonded magnet is raisedto an extremely high level.

[0005] At present, most of practically used high performance rare-earthbonded magnets are the isotropic bonded magnets which are made using theMQP-B powder manufactured by MQI Inc. as the rare-earth magnetic powderthereof. The isotropic bonded magnets are superior to the anisotropicbonded magnets in the following respect; namely, in the manufacture ofthe bonded magnet, the manufacturing process can be simplified becauseno magnetic field orientation is required, and as a result, the rise inthe manufacturing cost can be restrained. On the other hand, however,the conventional isotropic bonded magnets represented by thosemanufactured using the MQP-B powder involve the following problems.

[0006] The conventional isotropic bonded magnets do not have asufficiently high magnetic flux density. Namely, because the magneticpowder that has been used has poor magnetization, the content of themagnetic powder to be contained in the bonded magnet has to beincreased. However, the increase in the content of the magnetic powderleads to the deterioration in the moldability of the bonded magnet, sothere is a certain limit in this attempt. Moreover, even if the contentof the magnetic powder is somehow managed to be increased by changingthe molding conditions or the like, there still exists a limit to theobtainable magnetic flux density. For these reasons, it is not possibleto reduce the size of the motor by using the conventional isotropicbonded magnets.

[0007] Although there are reports concerning nanocomposite magnetshaving high remanent magnetic flux densities, their coercive forces, onthe contrary, are so small that the magnetic flux densities (for thepermeance in the actual use) obtainable when they are practically usedin motors are very low. Further, these magnets have poor heat stabilitydue to their small coercive forces.

[0008] The conventional bonded magnets have low corrosion resistance andheat resistance. Namely, in these magnets, it is necessary to increasethe content of the magnetic powder to be contained in the bonded magnetin order to compensate the low magnetic properties (magneticperformance) of the magnetic powder. This means that the density of thebonded magnet becomes extremely high. As a result, the corrosionresistance and heat resistance of the bonded magnet are deteriorated,thus resulting in low reliability.

SUMMARY OF THE INVENTION

[0009] It is therefore an object of the present invention to providemagnetic powder that can manufacture bonded magnets having excellentmagnetic properties and having excellent reliability.

[0010] In order to achieve the above object, the present invention isdirected to magnetic powder composed of an alloy composition representedby R_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least onekind of rare-earth element excepting Dy, M is at least one kind ofelement selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is7.1-9.9 at %, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30),and the magnetic powder being constituted from a composite structurehaving a soft magnetic phase and a hard magnetic phase, wherein when themagnetic powder is mixed with a binding resin and then the mixture issubjected to injection molding or extrusion molding to form a bondedmagnet having a density p[Mg/m³], the maximum magnetic energy product(BH)_(max)[kJ/m³] of the bonded magnet at a room temperature satisfiesthe relationship represented by the formula of (BH)_(max)/ρ²[×10⁻⁹J·m³/g²]≧2.10, and the intrinsic coercive force H_(CJ) of the bondedmagnet at a room temperature is in the range of 400-760 kA/m.

[0011] According to the magnetic powder as described above, it ispossible to provide bonded magnets having excellent magnetic propertiesas well as excellent reliability.

[0012] In this magnetic powder, it is preferred that the remanentmagnetic flux density Br[T] of the bonded magnet at a room temperaturesatisfies the relationship represented by the formula of Br/ρ[×10⁻⁶T·m³/g]≧0.125. This makes it possible to provide bonded magnets havingespecially excellent magnetic properties and reliability.

[0013] Another aspect of the present invention is also directed to amagnetic powder composed of an alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder being constituted from a composite structure having asoft magnetic phase and a hard magnetic phase, wherein when the magneticpowder is mixed with a binding resin and then the mixture is subjectedto injection molding or extrusion molding to form a bonded magnet havinga density ρ[Mg/m³], the remanent magnetic flux density Br[T] of thebonded magnet at a room temperature satisfies the relationshiprepresented by the formula of Br/ρ[×10⁻⁶ T·m³/g]>0.125 and the intrinsiccoercive force H_(CJ) of the bonded magnet at a room temperature is inthe range of 400-760 kA/m.

[0014] According to the magnetic powder as described above, it is alsopossible to provide bonded magnets having excellent magnetic propertiesas well as excellent reliability.

[0015] In the present invention, it is preferred that the magneticpowder is obtained by milling a melt spun ribbon. This makes it possibleto further improve magnetic properties, especially coercive force andthe like.

[0016] Further, it is also preferred that the thickness of the melt spunribbon is 10-40 μm. This also makes it possible to obtain bonded magnetshaving especially excellent magnetic properties.

[0017] Preferably, the melt spun ribbon is obtained by colliding amolten alloy of a magnetic material onto a circumferential surface of acooling roll which is rotating to cool and then solidify it. Accordingto this method, it is possible to obtain microstructure (fine crystalgrains) with relative ease, so that the magnetic properties can befurther improved.

[0018] In this case, it is preferred that the cooling roll includes aroll base made of a metal or an alloy and an outer surface layerprovided on an outer peripheral portion of the roll base to constitutethe circumferential surface, in which the outer surface layer of thecooling roll has a heat conductivity lower than the heat conductivity ofthe toll base. This makes it possible to quench the puddle of themagnetic material with an adequate cooling rate, so that it becomespossible to obtain magnets having especially excellent magneticproperties.

[0019] In this case, it is preferred that the outer surface layer of thecooling roll is formed of a ceramics. This also makes it possible toquench the puddle of the magnetic material with an adequate coolingrate, so that it becomes possible to obtain magnets having especiallyexcellent magnetic properties. Further, the durability of the coolingroll is also improved.

[0020] In the present invention, it is preferred that the R comprisesrare-earth elements mainly containing Nd and/or Pr. This makes itpossible to improve saturation magnetization of the hard phase of thecomposite structure (in particular, nanocomposite structure), andthereby the coercive force is further enhanced.

[0021] Further, it is also preferred that said R includes Pr and itsratio with respect to the total mass of said R is 5-75%. This makes itpossible to improve the coercive force and rectangularity withoutlowering the remanent magnetic flux density.

[0022] In the present invention, it is also preferred that the compositestructure includes a nanocomposite structure. This makes it possible toimprove magnetizability as well as heat resistance (heat stability) sothat changes in the magnetic properties with the elapse of time becomesmall.

[0023] Further, it is also preferred that the magnetic powder issubjected to a heat treatment for at least once during the manufacturingprocess or after its manufacturing. According to this, homogeneity(uniformity) of the structure can be obtained and influence of stressintroduced by the milling process can be removed, thereby enabling tofurther improve the magnetic properties of the bonded magnet.

[0024] In the magnetic powders described above, it is preferred that themean crystal grain size is 5-50 nm. This makes it possible to providemagnets having excellent magnetic properties, especially excellentcoercive force and rectangularity.

[0025] Further, in the magnetic powders described above, it is alsopreferred that the average particle size lies in the range of 0.5-150μm. This makes it possible to further improve the magnetic properties.Further, when the magnetic powder is used in manufacturing bondedmagnets, it is possible to obtain bonded magnets having a high contentof the magnetic powder and having excellent magnetic properties.

[0026] Further, the present invention is directed to a method ofmanufacturing magnetic powder, in which a melt spun ribbon is obtainedby colliding a molten alloy of a magnetic material onto acircumferential surface of a cooling roll which is rotating to cool andthen solidify it, and then thus obtained melt spun ribbon is milled toobtain the magnetic powder, in which the magnetic powder being composedof an alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder being constituted from a composite structure having asoft magnetic phase and a hard magnetic phase, wherein when the magneticpowder is mixed with a binding resin and then the mixture is subjectedto injection molding or extrusion molding to form a bonded magnet havinga density ρ[Mg/m³], the maximum magnetic energy product(BH)_(max)[kJ/m³] of the bonded magnet at a room temperature satisfiesthe relationship represented by the formula of (BH)_(max)/ρ²[×10⁻⁹J·m³/g²]≧2.10, and the intrinsic coercive force H_(CJ) of the bondedmagnet at a room temperature is in the range of 400-760 kA/m.

[0027] According to this method, it is possible to provide magneticpowder having excellent magnetic properties and having excellentreliability.

[0028] Further, the present invention is also directed to a method ofmanufacturing magnetic powder, in which a melt spun ribbon is obtainedby colliding a molten alloy of a magnetic material onto acircumferential surface of a cooling roll which is rotating to cool andthen solidify it, and then thus obtained melt spun ribbon is milled toobtain the magnetic powder, in which the magnetic powder being composedof an alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder being constituted from a composite structure having asoft magnetic phase and a hard magnetic phase, wherein when the magneticpowder is mixed with a binding resin and then the mixture is subjectedto injection molding or extrusion molding to form a bonded magnet havinga density ρ[Mg/m³], the remanent magnetic flux density Br[T] of thebonded magnet at a room temperature satisfies the relationshiprepresented by the formula of Br/ρ[×10⁻⁶ T·m³/g]≧0.125 and the intrinsiccoercive force H_(CJ) of the bonded magnet at a room temperature is inthe range of 400-760 kA/m.

[0029] According to this method, it is also possible to provide magneticpowder having excellent magnetic properties and having excellentreliability.

[0030] Furthermore, the present invention is directed to a bonded magnetmanufactured by mixing magnetic powder with a binding resin and thensubjecting the mixture to injection molding or extrusion molding, inwhich the magnetic powder being composed of an R-TM-B based alloy havingat least one element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr andDy (where R is at least one kind of rare-earth element excepting Dy, andTM is a transition metal mainly containing Fe), the bonded magnet beingcharacterized in that when a density of the bonded magnet is p[Mg/m³],the maximum magnetic energy product (BH)_(max)[kJ/m³] of the bondedmagnet at a room temperature satisfies the relationship represented bythe formula of (BH)_(max)/ρ²[×10⁻⁹ J·m³/g²]≧2.10, and the intrinsiccoercive force H_(CJ) of the bonded magnet at a room temperature is inthe range of 400-760 kA/m.

[0031] According to the bonded magnet described above, it is possible toobtain bonded magnets having excellent magnetic properties and havingexcellent reliability.

[0032] In this bonded magnet, it is preferred that the remanent magneticflux density Br[T] of the bonded magnet at a room temperature satisfiesthe relationship represented by the formula of Br/ρ [×10⁻⁶T·m³/g]≧0.125. This makes it possible to provide bonded magnets havingespecially excellent magnetic properties and reliability.

[0033] Further, the present invention is directed to a bonded magnetmanufactured by mixing magnetic powder with a binding resin, and thensubjecting the mixture to injection molding or extrusion molding, inwhich the magnetic powder being composed of an R-TM-B based alloy havingat least one element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr andDy (where R is at least one kind of rare-earth element excepting Dy, andTM is a transition metal mainly containing Fe), the bonded magnet beingcharacterized in that when the density of the bonded magnet is p[Mg/m³],the remanent magnetic flux density Br[T] of the bonded magnet at a roomtemperature satisfies the relationship represented by the formula ofBr/ρ[×10⁻⁶ T·m³/g]≧0.125, and the intrinsic coercive force H_(CJ) of thebonded magnet at a room temperature is in the range of 400-760 kA/m.

[0034] According to the bonded magnet described above, it is alsopossible to obtain bonded magnets having excellent magnetic propertiesand having excellent reliability.

[0035] In these bonded magnets, it is preferred that the magnetic powderis composed of an alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder is constituted from a composite structure having a softmagnetic phase and a hard magnetic phase. This also makes it possible toprovide bonded magnets having especially excellent magnetic propertiesand reliabilities.

[0036] Furthermore, it is also preferred that the maximum magneticenergy product (BH)_(max)[kJ/m³] is equal to or greater than 40 kJ/m³.This makes it possible to obtain small sized and high performancemotors.

[0037] Moreover, it is also preferred that the absolute value of theirreversible flux loss (initial flux loss) is less than 6.2%. This makesit possible to obtain bonded magnets having especially excellent heatresistance (heat stability)

[0038] These and other objects, structures and advantages of the presentinvention will be apparent from the following detailed description ofthe invention and the examples thereof which proceeds with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is an illustration which schematically shows one example ofa composite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

[0040]FIG. 2 is an illustration which schematically shows anotherexample of a composite structure (nanocomposite structure) of magneticpowder according to the present invention.

[0041]FIG. 3 is an illustration which schematically shows other exampleof a composite structure (nanocomposite structure) of magnetic powderaccording to the present invention.

[0042]FIG. 4 is a perspective view which shows an example of theconfiguration of an apparatus (melt spinning apparatus) formanufacturing a magnetic material.

[0043]FIG. 5 is a sectional side view showing the situation in thevicinity of colliding section of a molten alloy with a cooling roll inthe apparatus shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

[0044] In the following, embodiments of the magnetic powder according tothis invention, the manufacturing method of the magnetic powder and thebonded magnet formed from the magnetic powder will be described indetail.

[0045] At present, a magnet having high magnetic flux density ispractically required in order to reduce the size of motors or otherelectrical devices. In bonded magnets, factors that determine themagnetic flux density are the magnetization of magnetic powder and thecontent (compositional ratio) of the magnetic powder to be contained inthe bonded magnet. When the magnetization of the magnetic powder itselfis not so high, a desired magnetic flux density cannot be obtainedunless the content of the magnetic powder in the bonded magnet isincreased to an extremely high level.

[0046] As described in the above, the MQP-B powder made by MQI Inc.which is now being widely used can not provide sufficient magnetic fluxdensity depending on its use. As a result, in manufacturing the bondedmagnets, it is required to increase the content of the magnetic powderin the bonded magnet, that is, it is required to increase the magneticflux density. However, this in turn leads to the lack of reliability inthe corrosion resistance, heat resistance and mechanical strengththereof and the like. Further, there is also a problem in that theobtained magnet has a poor magnetizability due to its high coerciveforce.

[0047] In contrast, the magnetic powder and the bonded magnet accordingto this invention can obtain a sufficient magnetic flux density and anadequate coercive force. As a result, without extremely increasing thecontent of the magnetic powder in the bonded magnet, it is possible toprovide a bonded magnet having high strength and having excellentmoldability, corrosion resistance and magnetizability. This makes itpossible to reduce the size of the bonded magnet and increase itsperformance, thereby contributing to reduction in size of motors andother devices employing magnets.

[0048] Further, the magnetic powder of the present invention may beformed so as to constitute a composite structure having a hard magneticphase and a soft magnetic phase.

[0049] While the MQP-B powder made by MQI Inc. described above is asingle phase structure of a hard magnetic phase, the magnetic powder ofthe present invention has the composite structure which also has a softmagnetic phase with high magnetization. Accordingly, the magnetic powderof the present invention has an advantage in that the totalmagnetization of the system as a whole is high. Further, since therecoil permeability of the bonded magnet becomes high, there is anadvantage in that, even after a reverse magnetic field has been applied,the demagnetization factor remains small.

[0050] The magnetic powder according to the present invention iscomposed of an alloy composition containing at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy. According to suchmagnetic powder, coercive force can be improved, and heat resistance andcorrosion resistance can also be improved. In particular, it ispreferable that the magnetic powder is composed of an R-TM-B based alloyhaving at least one element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn,Zr and Dy (where R is at least one kind of rare-earth element exceptingDy, and TM is a transition metal mainly containing Fe). Further, it ismore preferable that the magnetic powder is composed of an alloycomposition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30).

[0051] Examples of R (at least one kind of rare-earth element exceptingDy, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb, Lu, and amisch metal, and one or more of these rare-earth elements may becontained.

[0052] The content of R is in the range of 7.1-9.9 at %. When thecontent of R is less than 7.1 at %, sufficient coercive force can not beobtained, and even if M is added, improvement of the coercive force cannot be hardly attained. On the other hand, when the content of R exceeds9.9 at %, potential of magnetization is lowered, so that a sufficientmagnetic flux density can not be obtained.

[0053] Here, it is preferable that R includes the rare-earth elements Ndand/or Pr as its principal ingredient. The reason for this is that theserare-earth elements enhance the saturation magnetization of the hardmagnetic phase which constitutes the composite structure (especially,nanocomposite structure), and are effective in realizing a satisfactorycoercive force as a magnet.

[0054] Moreover, it is preferable that R includes Pr and its ratio tothe total mass of R is 5-75%, and more preferably 20-60%. This isbecause when the ratio lies within this range, it is possible to improvethe coercive force (coercivity) and the rectangularity by hardly causinga drop in the remanent magnetic flux density.

[0055] Cobalt (Co) is a transition metal element having propertiessimilar to Fe. By adding Co, that is by substituting a part of Fe by Co,the Curie temperature is elevated and the temperature characteristic ofthe magnetic powder is improved. However, if the substitution ratio ofFe by Co exceeds 0.30, both of the coercive force and the magnetic fluxdensity tend to fall off. The range of 0.05-0.20 of the substitutionratio of Fe by Co is more preferable since in this range not only thetemperature characteristic of the magnetic powder but also the magneticflux density thereof are improved.

[0056] Boron (B) is an element which is important for obtaining highmagnetic properties, and its content is set to 4.6-3.0 at %. If thecontent of B is less than 4.6 at %, the rectangularity of the B-H (J-H)loop is deteriorated. On the other hand, if the content of B exceeds 8.0at %, the nonmagnetic phase increases and thereby the magnetic fluxdensity drops sharply.

[0057] M is an element which is advantageous for enhancing the coerciveforce, and its content should lie in the range of 0.1-3.0 at %. In thisregard, it is preferable that the content of M is in the range of0.2-2.5 at %, and it is more preferable that the content is in the rangeof 0.5-2.0 at %. By containing M of such a range, prominent coerciveforce enhancement effect is realized. Further, containing M of such arange also improves the rectangularity and the maximum magnetic energyproduct together with the coercive force. Furthermore, heat resistanceand corrosion resistance are also improved. However, as described above,when the content of R is less than 7.1 at %, such an effect derived fromthe addition of M is very small. On the other hand, when the content ofM exceeds the above upper limit, magnetization is lowered.

[0058] Of course, M itself is a known substance. However, in the presentinvention, it has been found through repeatedly conducted experimentsand researches that by containing M within the above range to themagnetic powder constituted from a composite structure having a softmagnetic phase and a hard magnetic phase, the following three effectsare realized, in particular these three effects are realized at the sametime, and this is the significance of the present invention.

[0059] The coercive force of the magnetic powder can be improved whilemaintaining the excellent rectangularity and the maximum magnetic energyproduct.

[0060] The irreversible flux loss can be improved, that is the absolutevalue thereof can be lowered.

[0061] Better corrosion resistance can be maintained.

[0062] In addition, for the purpose of further improving the magneticproperties, at least one other element selected from the groupcomprising Ti, Zn, P, Ge, Cu, Ga, Si, In, Ag and Al (hereinafter,referred to as “Q”) may be contained as needed. When containing theelement belonging to Q, it is preferable that the content thereof shouldbe equal to or less than 2.0 at %, and it is more preferable that thecontent thereof lies within the range of 0.1-1.5 at %, and it isfurthermore preferable that the content thereof lies within the range of0.2-1.0 at %.

[0063] The addition of the element belonging to Q makes it possible toexhibit an inherent effect of the kind of the element. For example, theaddition of Ta, Cu, Ga, Si or Al exhibits an effect of improving thecorrosion resistance.

[0064] As described above, the magnetic material of the presentinvention has a composite structure having a soft magnetic phase and ahard magnetic phase.

[0065] In this composite structure (nanocomposite structure), a softmagnetic phase 10 and a hard magnetic phase 11 exist in a pattern(model) as shown in, for example, FIG. 1, FIG. 2 or FIG. 3, where thethickness of the respective phases and the grain size are on the orderof nanometers. Further, the soft magnetic phase 10 and the hard magneticphase 11 are arranged adjacent to each other (this also includes thecase where these phases are adjacent through intergranular phases),which makes it possible to perform magnetic exchange interactiontherebetween.

[0066] In such nanocomposite structure, it is preferable that the meancrystal grain size is 5 to 50 nm, and it is more preferable that themean crystal grain size is 10 to 40 nm. If the mean crystal grain sizeis less than the lower limit value, the influence of the magneticexchange interaction between the crystal grains too large, so thatreversal of magnetization becomes easy, thus leading to the case thatthe coercive force is deteriorated.

[0067] On the other hand, if the mean crystal grain size exceeds theabove upper limit, there is a case that the crystal grain size becomescoarse. Further, since the influence of the magnetic exchangeinteraction is weakened, there is a case that the magnetic flux density,coercive force, rectangularity and maximum energy product aredeteriorated.

[0068] In this regard, it is to be noted that the patterns illustratedin FIG. 1 to FIG. 3 are only specific examples, and are not limitedthereto. For example, the soft magnetic phase 10 and the hard magneticphase 11 in FIG. 2 are interchanged to each other.

[0069] The magnetization of the soft magnetic phase readily changes itsorientation by the action of an external magnetic field. Therefore, whenthe soft magnetic phase coexists with the hard magnetic phase, themagnetization curve for the entire system shows a stepped “serpentinecurve” in the second quadrant of the B-H diagram. However, when the softmagnetic phase has a sufficiently small size, magnetization of the softmagnetic phase is sufficiently and strongly constrained through thecoupling with the magnetization of the surrounding hard magnetic phase,so that the entire system exhibits functions like a hard magnetic phase.

[0070] A magnet having such a composite structure (nanocompositestructure) has mainly the following five features.

[0071] In the second quadrant of the B-H diagram (J-H diagram), themagnetization springs back reversively (in this sense, such a magnet isalso referred to as a “spring magnet”).

[0072] It has a satisfactory magnetizability, and it can be magnetizedwith a relatively low magnetic field.

[0073] The temperature dependence of the magnetic properties is small ascompared with the case where the system is constituted from a hardmagnetic phase alone.

[0074] The changes in the magnetic properties with the elapse of timeare small.

[0075] No deterioration in the magnetic properties is observable even ifit is finely milled.

[0076] In the alloy composition described in the above, the hardmagnetic phase and the soft magnetic phase are respectively composed ofthe followings, for instance.

[0077] The hard magnetic phase: R₂TM₁₄B system (where, TM is a transitmetal containing Fe or Fe and Co as its major components)

[0078] The soft magnetic phase: TM (α-Fe or α-(Fe, Co) in particular)

[0079] As for the magnetic powders according to this invention, it ispreferable that they are manufactured by quenching a molten alloy. Inthis case, it is more preferable that they are manufactured by milling amelt spun (quenched) ribbon obtained by quenching (cooling) the moltenalloy and then solidifying it. An example of such a method will bedescribed in the following.

[0080]FIG. 4 is a perspective view showing an example of the structureof an apparatus (melt spinning apparatus) for manufacturing a magneticmaterial by the quenching method using a single roll, and FIG. 5 is asectional side view showing the situation in the vicinity of collidingsection of the molten alloy with the cooling roll of the apparatus shownin FIG. 4.

[0081] As shown in FIG. 4, the melt spinning apparatus 1 is providedwith a cylindrical body 2 capable of storing the magnetic material, anda cooling roll 5 which rotates in the direction of an arrow 9A in thefigure relative to the cylindrical body 2. A nozzle (orifice) 3 whichinjects a molten alloy of the magnetic material is formed at the lowerend of the cylindrical body 2.

[0082] In addition, a heating coil 4 is arranged on the outer peripheryof the cylindrical body 2 in the vicinity of the nozzle 3, and themagnetic material in the cylindrical body 2 is melted by inductivelyheating the interior of the cylindrical body 2 through application of,for example, a high frequency wave to the coil 4.

[0083] The cooling roll 5 is constructed from a base part 51 and asurface layer 52 which forms a circumferential surface 53 of the coolingroll 5.

[0084] The base part 51 may be formed of the same material as that forthe surface layer 52. However, it is preferred that the surface layer 52is formed of a material having lower heat conductivity than that for thematerial for the base part 51.

[0085] Although there is no particular limitation on the material usedfor the base part 51, it is preferable that the base part 51 is formedof a metallic material with high heat conductivity such as copper or acopper alloy in order to make it possible to dissipate heat of thesurface layer 52 as quickly as possible.

[0086] Examples of the surface layer 52 include a metallic thin layer ofCr, Ni, Pd, w or the like, a layer of metallic oxide of these metals anda ceramic layer. Among these layers, a ceramic layer is particularlypreferred, since such ceramic layer makes it possible to reduce thedifference in the cooling rates at the roll contact surface 81 of themelt spun ribbon 8 and at the free surface 82 thereof. Here, it is to benoted that the roll contact surface of the melt spun ribbon 8 means asurface of the melt spun ribbon 8 which is in contact with the coolingroll 5, and the free surface means the opposite surface of the rollcontact surface.

[0087] Examples of the ceramics to be used for the ceramic layer includeoxide ceramics such as Al₂O₃, SiO₂, TiO₂, Ti₂O₃, ZrO₂, Y₂O₃, bariumtitanate and strontium tinanate and the like; nitride ceramics such asAlN, Si₃N₄, TiN and BN and the like; carbide ceramics such as graphite,SiC, ZrC, Al₄C₃, CaC₂ and WC and the like; and mixture of two or more ofthese ceramics (composite ceramics)

[0088] The surface layer 52 may be formed from a laminate structurecomprised of a plurality of layers of different compositions, besidesthe single layer structure described above. In this case, it ispreferred that the adjacent layers are well adhered or bonded to eachother. For this purpose, these adjacent layers may contain the sameelement therein.

[0089] Further in the case where the surface layer 52 is formed into thesingle layer structure described above, it is not necessary for thecomposition of the material of the surface layer to have uniformdistribution in the thickness direction thereof. For example, thecontents of the constituents may be gradually changed in the thicknessdirection thereof (that is, graded materials may be used).

[0090] The average thickness of the surface layer 52 (in the case of thelaminate structure, the total thickness thereof) is not limited to aspecific value. However, it is preferred that the average thickness lieswithin the range of 0.5-50 μm, and more preferably 1-20 μm.

[0091] If the average thickness of the surface layer 52 is less than thelower limit value described above, there is a possibility that thefollowing problems will be raised. Namely, depending on the material tobe used for the surface layer 52, there is a case that cooling abilitybecomes too high. When such a material is used for the surface layer 52,a cooling rate becomes too large in the vicinity of the roll contactsurface 81 of the melt spun ribbon 8 even though it has a considerablylarge thickness, thus resulting in the case that amorphous structure beproduced at that portion. On the other hand, in the vicinity of the freesurface 82 of the spun ribbon 8 where the heat conductivity isrelatively low, the cooling rate becomes small as the thickness of themelt spun ribbon 8 increases, so that crystal grain size is liable to becoarse. Namely, in this case, the grain size is liable to be coarse inthe vicinity of the free surface 82 of the obtained melt spun ribbon 8,and amorphous structure is liable to be produced in the vicinity of theroll contact surface 81 of the melt spun ribbon 8, which result in thecase that satisfactory magnetic properties can not be obtained. In thisregard, even if the thickness of the melt spun ribbon 8 is made small byincreasing the peripheral velocity of the cooling roll 5, for example,in order to reduce the crystal grain size in the vicinity of the freesurface 82 of the melt spun ribbon 8, this in turn leads to the casethat the melt spun ribbon 8 has more random amorphous structure in thevicinity of the roll contact surface 81 of the obtained melt spun ribbon8. In such a melt spun ribbon 8, there is a case that sufficientmagnetic properties will not be obtained even if it is subjected to aheat treatment after manufacturing thereof.

[0092] Further, if the average thickness of the surface layer 52 exceedsthe above upper limit value, the cooling rate becomes slow and therebythe crystal grain size becomes coarse, thus resulting in the case thatmagnetic properties are poor.

[0093] The melt spinning apparatus 1 described above is installed in achamber (not shown), and it is operated preferably under the conditionwhere the interior of the chamber is filled with an inert gas or otherkind of gas. In particular, in order to prevent oxidation of a melt spunribbon 8, it is preferable that the gas is an inert gas such as argongas, helium gas, nitrogen gas or the like.

[0094] In the melt spinning apparatus 1, the magnetic material (alloy)is placed in the cylindrical body 2 and then melted by heating with thecoil 4, and the molten alloy 6 is discharged from the nozzle 3. Then, asshown in FIG. 5, the molten alloy 6 collides with the circumferentialsurface 53 of the cooling roll 5, and after the formation of a puddle 7,the molten alloy 6 is cooled down rapidly to be solidified while draggedalong the circumferential surface 53 of the rotating cooling roll 5,thereby forming a melt spun ribbon 8 continuously or intermittently. Aroll surface 81 of the melt spun ribbon 8 thus formed is soon releasedfrom the circumferential surface 53, and the melt spun ribbon 8 proceedsin the direction of an arrow 9B in FIG. 4. The solidification interface71 of the molten alloy is indicated by a broken line in FIG. 5.

[0095] The optimum range of the peripheral velocity of the cooling roll5 depends upon the composition of the molten alloy, the structuralmaterial (composition) of the surface layer 52, and the surfacecondition of the circumferential surface 53 (especially, the wettabilityof the surface layer 52 with respect to the molten alloy 6), and thelike. However, for the enhancement of the magnetic properties, aperipheral velocity in the range of 5 to 60 m/s is normally preferable,and 10 to 40 m/s is more preferable. If the peripheral velocity of thecooling roll 5 is less than the above lower limit value, the coolingrate of the molten alloy 6 (puddle 7) is decreased. This tends toincrease the crystal grain sizes, thus leading to the case that themagnetic properties are lowered. On the other hand, when the peripheralvelocity of the cooling roll 5 exceeds the above upper limit value, thecooling rate is too high, and thereby amorphous structure becomesdominant. In this case, the magnetic properties can not be sufficientlyimproved even if a heat treatment described below is given in the laterstage.

[0096] It is preferred that thus obtained melt spun ribbon 8 has uniformwidth w and thickness t. In this case, the average thickness t of themelt spun ribbon 8 should preferably lie in the range of 10-40 μm andmore preferably lie in the range of 12-30 μm. If the average thickness tis less than the lower limit value, amorphous structure becomesdominant, so that there is a case that the magnetic properties can notbe sufficiently improved even if a heat treatment is given in the laterstage. Further, productivity per an unit time is also lowered. On theother hand, if the average thickness t exceeds the above upper limitvalue, the crystal grain size at the side of the free surface 82 of themelt spun ribbon 8 tends to be coarse, so that there is a case that themagnetic properties are lowered.

[0097] Further, the obtained melt spun ribbon 8 may be subjected to atleast one heat treatment for the purpose of, for example, accelerationof recrystallization of the amorphous structure and homogenization ofthe structure. The conditions of this heat treatment may be, forexample, a heating at a temperature in the range of 400 to 900° C. for0.2 to 300 min.

[0098] Moreover, in order to prevent oxidation, it is preferred thatthis heat treatment is performed in a vacuum or under a reduced pressure(for example, in the range of 1×10⁻¹ to 1×10⁻⁶ Torr), or in anonoxidizing atmosphere of an inert gas such as nitrogen gas, argon gas,helium gas or the like.

[0099] The melt spun ribbon (ribbon-shaped magnetic material) 8 obtainedaccording to the manufacturing method as described above has amicrocrystalline structure or a structure in which microcrystals areincluded in an amorphous structure, and exhibits excellent magneticproperties. The magnetic powder of the present invention is obtained bymilling the thus manufactured melt spun ribbon 8.

[0100] The milling method of the melt spun ribbon is not particularlylimited, and various kinds of milling or crushing apparatus such as ballmill, vibration mill, jet mill, and pin mill may be employed. In thiscase, in order to prevent oxidation, the milling process may be carriedout under vacuum or reduced pressure (for example, under a reducepressure of 1×10⁻¹ to 1×10⁻⁶ Torr), or in a nonoxidizing atmosphere ofan inert gas such as nitrogen, argon, helium, or the like.

[0101] The mean particle size of the magnetic powder is not particularlylimited. However, in the case where the magnetic powder is used formanufacturing bonded magnets described later, in order to preventoxidation of the magnetic powder and deterioration of the magneticproperties during the milling process, it is preferred that the meanparticle size lies in the range of 0.5 to 150 μm, more preferably in therange of 0.5 to 80 μm, and still more preferably in the range of 1 to 50μm.

[0102] In order to obtain a better moldability during the manufacturingprocess of the bonded magnet, it is preferable to give a certain degreeof dispersion to the particle size distribution of the magnetic powder.By so doing, it is possible to reduce the void ratio (porosity) of thebonded magnet obtained. As a result, it is possible to raise the densityand the mechanical strength of the bonded magnet as compared with otherbonded magnet containing the same amount of the magnetic powder, therebyenabling to further improve the magnetic properties.

[0103] Thus obtained magnetic powder may be subjected to a heattreatment for the purpose of, for example, removing the influence ofstress introduced by the milling process and controlling the crystalgrain size. The conditions of the heat treatment are, for example,heating at a temperature in the range of 350 to 850° C. for 0.2 to 300min.

[0104] In order to prevent oxidation of the magnetic powder, it ispreferable to perform the heat treatment in a vacuum or under a reducedpressure (for example, in the range of 1×10⁻¹ to 1×10⁻⁶ Torr), or in anonoxidizing atmosphere of an inert gas such as nitrogen gas, argon gas,and helium gas.

[0105] Thus obtained magnetic powder has a satisfactory bindability withthe binding resin (wettability of the binding resin). Therefore, when abonded magnet is manufactured using the magnetic powder described above,the bonded magnet has a high mechanical strength and excellent heatstability (heat resistance) and corrosion resistance. Consequently, itcan be concluded that the magnetic powder of the present invention issuitable for manufacture of bonded magnets.

[0106] In the above, the quenching method is described in terms of thesingle roll method, but the twin roll method may also be employed.Besides, other methods such as the atomizing method which uses gasatomization, the rotating disk method, the melt extraction method, andthe mechanical alloying method (MA) may also be employed. Since such amelt spinning method can refine the metallic structure (crystal grains),it is effective for enhancing the magnetic properties, especially thecoercive force or the like, of bonded magnets.

[0107] Next, a description will be made with regard to the bonded magnetaccording to the present invention.

[0108] The bonded magnet of this invention is manufactured by mixing themagnetic powder with a binding resin and then subjecting the mixture toinjection molding or extrusion molding.

[0109] As for the binding resin (binder), thermoplastic resins aremainly employed.

[0110] Examples of the thermoplastic resins include polyamid (example:nylon 6, nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12,nylon 6-12, nylon 6-66); thermoplastic polyimide; liquid crystal polymersuch as aromatic polyester; poly phenylene oxide; poly phenylenesulfide; polyolefin such as polyethylene, polypropylene andethylene-vinyl acetate copolymer; modified polyolefin; polycarbonate;poly methyl methacrylate; polyester such as poly ethylen terephthalateand poly butylene terephthalate; polyether; polyether ether ketone;polyetherimide; polyacetal; and copolymer, blended body, and polymeralloy having at least one of these materials as a main ingredient. Inthis case, a mixture of two or more kinds of these materials may beemployed.

[0111] Among these resins, a resin containing polyamide as its mainingredient is particularly preferred from the viewpoint of capable ofobtaining especially excellent moldability and high mechanical strength.Further, a resin containing liquid crystal polymer and/or poly phenylenesulfide as its main ingredient is also preferred from the viewpoint ofcapable of improvement of the heat resistance. Furthermore, thesethermoplastic resins also have an excellent kneadability with themagnetic powder.

[0112] These thermoplastic resins provide an advantage in that a widerange of selection can be made. For example, it is possible to use athermoplastic resin having a good moldability or to use a thermoplasticresin having good heat resistance and mechanical strength byappropriately selecting their kinds, copolymerization or the like.

[0113] The bonded magnet according to this invention described in theabove may be manufactured, for example, as in the following. First, themagnetic powder, a binding resin and an additive (antioxidant, lubricantor the like) as needed are mixed and kneaded (warm kneading) to form abonded magnet composite (compound). Then, thus obtained bonded magnetcomposite is formed into a desired shape of a bonded magnet in a statefree from magnetic field by means of injection molding or extrusionmolding.

[0114] According to the injection molding, a wide variety of selectionscan be made with regard to shapes of bonded magnets to be manufactured,and in particular it has an advantage in that even bonded magnets havingrelatively complicated shapes can be manufactured with ease. Further, asis the same with the injection molding, use of the extrusion moldingalso has advantages in that a wide variety of selections can be madewith regard to shapes of bonded magnets to be manufactured and in thatit can realize high productivity.

[0115] However, these molding methods in turn require that the compoundshave sufficient fluidity within molding machines in order to secure goodmoldability. Therefore, when these methods are used, it is difficult toincrease the content of the magnetic powder as compared with the casewhere the compaction molding is used. In other words, when these moldingmethods are used, it is difficult to make bonded magnets high densities.

[0116] However, as stated in the foregoing, the magnetic powderaccording to the present invention has extremely high magneticproperties as compared with the conventional magnetic powders.Therefore, if bonded magnets are manufactured using the magnetic powderof the present invention by means of these molding methods (that is,injection molding or extrusion molding), it is possible to obtainmagnetic properties that are equivalent to or higher than those of theconventional bonded magnets manufactured by means of the compressionmolding, without impairing the excellent moldability and productivity.

[0117] The content of the magnetic powder contained in the bonded magnetis not limited to a specific value, but preferably it should lie in therange of 75-98 wt %, and more preferably in the range of 85-97 wt %.

[0118] The density ρ of the bonded magnet is determined by factors suchas the specific gravity of the magnetic powder contained in the magnetand the content of the magnetic powder, and void ratio (porosity) of thebonded magnet and the like. In the bonded magnets according to thisinvention, the density ρ is not particularly limited to a specificvalue, but it is preferable to be in the range of 4.3-6.3 Mg/m³, andmore preferably in the range of 4.8-6.2 Mg/m³.

[0119] As described above, in the present invention, even if the amountof the magnetic powder to be contained in the bonded magnet is decreasedin order to improve moldability upon molding processes, it is stillpossible to obtain a bonded magnet having excellent magnetic propertiesdue to the high magnetic properties of the magnetic powder itself.

[0120] Further, the shape, dimensions, and the like of the bonded magnetmanufactured according to this invention are not particularly limited.For example, as to the shape, all shapes such as columnar, prism-like,cylindrical (ring-shaped), circular, plate-like, curved plate-likeshapes, and the like are acceptable. As to the dimensions, all sizesstarting from large-sized one to ultraminuaturized one are acceptable.However, as repeatedly described in this specification, the presentinvention is particularly advantageous in miniaturization andultraminiaturization of the bonded magnet.

[0121] In view of the advantages described above, it is preferred thatthe bonded magnet of the present invention is subject to multipolarmagnetization so as to have multipoles.

[0122] Furthermore, in the present invention, it is preferred that thebonded magnet can satisfy the following conditions.

[0123] The coercive force (H_(CJ)) of the bonded magnet (that is, theintrinsic coercive force at a room temperature) should lie in the rangeof 400 to 760 kA/m. In this case, it is preferred that the coerciveforce lies in the range of 430 to 720 kA/m. If the coercive force islower than the lower limit value, demagnetization occurs conspicuouslywhen a reverse magnetic field is applied depending upon the usage of themotor, and the heat resistance at a high temperature is deteriorated. Onthe other hand, if the coercive force exceeds the above upper limitvalue, magnetizability is deteriorated. Therefore, by setting thecoercive force (H_(CJ)) to the above range, in the case where the bondedmagnet (cylindrical magnet in particular) is subjected to multipolarmagnetization, a satisfactory magnetization with a sufficiently highmagnetic flux density can be accomplished even when a sufficiently highmagnetizing field cannot be secured, which makes it possible to providehigh performance bonded magnets, especially high performance bondedmagnets for motors.

[0124] Preferably, the bonded magnets should satisfy the followingformula (I) between the maximum magnetic energy product (BH)_(max) andthe density ρ (Mg/m³).

2.10≦(BH)_(max)/ρ²[×10⁻⁹ J·m ³ /g ²]  (I)

[0125] In this connection, it is more preferable that the followingformula (II) is satisfied between the maximum magnetic energy product(BH)_(max) and the density ρ (Mg/m³) instead of the above formula (1),and it is most preferable that the following formula (III) is satisfiedtherebetween.

2.2≦(BH)_(max)/ρ² [×10⁻⁹ J·m ³ /g ^(2]≦)3.2  (II)

2.3≦(BH)_(max)/ρ²[×10⁻⁹ J·m ³ /g ²]≦3.1  (III)

[0126] When the value of (BH)_(max)/ρ²[×10⁻⁹ J·m³/g²] is less than thelower limit value of the above formulas, it is not possible to obtainsufficient magnetic properties unless otherwise the density of themagnet is increased, that is the content of the magnetic powder in themagnet is increased. However, this in turn leads to problems in thatavailable molding methods are limited, manufacturing cost is increased,and moldability is lowered due to a reduced amount of the binding resin.Further, when magnetic properties of a certain level are to be obtained,a volume (size) of the magnet is necessarily increased, which results indifficulty in miniaturizing devices such as motors.

[0127] The bonded magnet should satisfy the following formula (IV)between the remanent magnetic flux density Br(T) and the density p(Mg/m³)

0.125≦Br/ρ[×10⁻⁶ T·m ³ /g]  (IV)

[0128] In this connection, it is more preferable that the followingformula (V) is satisfied between the remanent magnetic flux densityBr(T) and the density ρ (Mg/m³), and it is most preferable that thefollowing formula (VI) is satisfied therebetween.

0.128≦Br/ρ[×10⁻⁶ T·m ³ /g]≦0.16  (V)

0.13≦Br/ρ[×10⁻⁶ T·m ³ /g]≦0.155  (VI)

[0129] When the value of Br/ρ[×10⁻⁶ T·m³/g] is less than the lower limitvalue of the formula (I), it is not possible to obtain a sufficientmagnetic flux density unless otherwise the density of the magnet isincreased, that is the content of the magnetic powder in the magnet isincreased. However, this in turn leads to problems in that manufacturingcost is increased and moldability is lowered due to a reduced amount ofthe binding resin. Further, when a magnetic flux density of a certainlevel is to be obtained, a volume of the magnet is necessarilyincreased, which results in difficulty in miniaturizing devices such asmotors.

[0130] The maximum magnetic energy product (BH)_(max) of the bondedmagnet should preferably be equal to or greater than 40 kJ/m³, morepreferably be equal to or greater than 50 kJ/m³, and most preferably liein the range of 60 to 110 kJ/m³. If the maximum magnetic energy product(BH)_(max) is less than 40 kJ/m³, there is a case that sufficient torquewill not be obtained depending upon the kind and structure when used formotors.

[0131] It is preferable that the absolute value of the irreversible fluxloss (that is, initial flux loss) is equal to or less than 6.2%, it ismore preferable that the absolute value is equal to or less than 5.0%,and it is the most preferable that the absolute value is equal to orless than 4.0%. This makes it possible to obtain a bonded magnet havingexcellent heat stability (heat resistance).

EXAMPLES

[0132] Hereinbelow, the actual examples of the present invention will bedescribed.

Example 1

[0133] Seven types of magnetic powders respectively having the alloycompositions shown in the attached TABLE 1 (sample Nos. 1 to 7) wereobtained by the following method.

[0134] First, each of the materials Nd, Pr, Fe, Co, B and M was weighed,and then they were cast to produce a mother alloy ingot.

[0135] A melt spinning apparatus 1 as shown in FIG. 4 and FIG. 5 wasprepared, and the mother alloy ingot was placed in a quartz tube 2having a nozzle 3 (having a circular orifice of which diameter is 0.6mm) at the bottom. After evacuating the interior of a chamber in whichthe melt spinning apparatus 1 is installed, an inert gas (Ar gas) wasintroduced to obtain an atmosphere with desired temperature andpressure.

[0136] The cooling roll 5 of the melt spinning apparatus 1 was providedwith a surface layer 52 on the outer periphery of the base part 51 madeof Cu. The surface layer 52 was formed of ZrC and had a thickness ofabout 7 μm. The diameter of the cooling roll 5 was 200 mm.

[0137] Then, the ingot sample in the quartz tube 2 was melted by highfrequency induction heating. Further, the injecting pressure(differential pressure between the pressure of the atmosphere and thesummed pressure of the inner pressure of the quartz tube 2 and thepressure that is exerted in proportion to the surface level of themolten alloy) and the circumferential velocity were adjusted, therebyobtaining a melt spun ribbon. The thickness of thus obtained melt spunribbon was about 20 μm.

[0138] The melt spun ribbon was then coarsely crushed, and the powderwas subjected to a heat treatment in an argon gas atmosphere at 680° C.for 300 sec. In this way, the seven types of magnetic powders wereobtained.

[0139] Next, for the purpose of adjustment of the particle size, eachmagnetic powder was milled by a milling machine (in an argon gasatmosphere. In this way, magnetic powders of the samples Nos. 1 to 7each having an average particle diameter of 60 μm were obtained.

[0140] To analyze the phase structure of the obtained magnetic powders,each of the magnetic powders was subjected to X-ray diffraction usingCu—K line at the diffraction angle of 20°-60°. From the thus obtaineddiffraction pattern, the presence of diffracted peaks of a hard magneticphase, R₂(Fe.Co)₁₄B phase, and a soft magnetic phase, α-(Fe,Co) phase,were confirmed. Further, from the observation result using atransmission electron microscope (TEM), the formation of a compositestructure (nanocomposite structure) was confirmed in each magneticpowder. Furthermore, in each of the magnetic powders, the mean crystalgrain size thereof was measured. The results of these analysis andmeasurements are shown in the attached TABLE 1.

[0141] Next, each of the magnetic powders was mixed with a polyamideresin (Nylon 12) and then they were kneaded at a temperature of 225° C.for 15 min, to obtain a composite (compound) for a bonded magnet. Inthis case, the compounding ratio (mixing ratio by weight) of themagnetic powder with respect to the polyamide resin was common to therespective bonded magnets. Specifically, in each of the bonded magnets,the content of the magnetic powder was about 95 wt %.

[0142] Then, each of the thus obtained compounds was crushed to begranular, and then the granular substance was subjected to injectionmolding using the injection molding machine (“J50-E2”, a product of TheJapan Steel Works, Ltd.). At this time, the temperature of the die was90° C. and the temperature inside the injection cylinder was 240° C.After the molding was cooled, it was removed from the die. In this way,bonded magnets each having a columnar shape having a diameter of 10 mmand a height of 7 mm were obtained.

[0143] After pulse magnetization was performed for each of these bondedmagnets under the magnetic field strength of 3.2 MA/m, magneticproperties (remanent magnetic flux density Br, intrinsic coercive force(H_(CJ)), and maximum magnetic energy product (BH)_(max)) were measuredusing a DC recording fluxmeter (manufactured and sold by Toei IndustryCo. Ltd under the product code of TRF-5BH) under the maximum appliedmagnetic field of 2.0MA/m. The temperature at the measurement was 23° C.(that is, a room temperature).

[0144] Next, the heat resistance (heat stability) of each of the bondedmagnets was examined. The heat resistance was obtained by measuring theirreversible flux loss (initial flux loss) obtained when the bondedmagnet was being left in the atmosphere of 100° C. for one hour and thenthe temperature was lowered to a room temperature, and then it wasevaluated. In this connection, it is to be noted that smaller absolutevalue of the irreversible flux loss (initial flux loss) means moreexcellent heat resistance (heat stability).

[0145] The density ρ of each of the bonded magnets was also measured bythe Archimedean principle.

[0146] The results of these measurements and the values of (BH)_(max)/ρ²and Br/ρ are shown in the attached TABLE 2.

Example 2

[0147] Each of the magnetic powders which were obtained in the Example 1was mixed with a polyamide resin (Nylon 12) and then they were kneadedat a temperature of 225° C. for 15 min, to obtain a composite (compound)for a bonded magnet. In this case, the compounding ratio (mixing ratioby weight) of the magnetic powder with respect to the polyamide resinwas common to the respective bonded magnets. Specifically, in each ofthe bonded magnets, the content of the magnetic powder was about 96.5 wt%.

[0148] Next, each of the thus obtained compounds was crushed to begranular, and then the granular substance was subjected to extrusionmolding using an extrusion molding machine to obtain a molding. Then,the molding was cut into a piece of a predetermined length. In this way,bonded magnet each having a columnar shape having a diameter of 10 mmand a height of 7 mm were obtained. In this regard, the temperature ofthe die upon the extrusion molding was 150° C.

[0149] For each of these thus obtained bonded magnets, its magneticproperties, irreversible flux loss and magnetic flux density weremeasured in the same manner as Example 1.

[0150] The results of these measurements and the values of (BH)_(max)/ρ²and Br/ρ are shown in the attached TABLE 3.

[0151] As seen from TABLES 2 and 3, each of the bonded magnets of thesample No. 2 to No. 6 (which are the bonded magnets according to thepresent invention) have excellent magnetic properties (that is,excellent remanent magnetic flux density Br, maximum magnetic energyproduct (BH)_(max)and intrinsic coercive force H_(CJ)) irrespective ofthe molding methods (that is, irrespective of either of the injectionmolding or extrusion molding). Further, each of these bonded magnetsalso has a small absolute value of the irreversible flux loss so thatthe heat stability (heat resistance) of each bonded magnet is excellent.

[0152] In contrast, each of the bonded magnets of the sample Nos. 1 and7 (which are the bonded magnets according to Comparative Examples)exhibits poor magnetic properties and has a large absolute value of theirreversible flux loss so that the heat stability of these magnets islow.

[0153] As described above, the bonded magnets which are manufacturedusing the magnetic powders containing a predetermined amount of M haveexcellent magnetic properties and heat stability (heat resistance).

[0154] As described above, according to the present invention, thefollowing effects can be obtained.

[0155] Since the magnetic powder contains a predetermined amount of M(at least one element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr andDy) and has a composite structure having a soft magnetic phase and ahard magnetic phase, it can have high magnetization and exhibitexcellent magnetic properties. In particular, intrinsic coercive forceand rectangularity thereof are improved.

[0156] By appropriately selecting a combination of the elements M to becontained and the contents thereof, it is possible to obtain moreexcellent magnetic properties and heat resistance.

[0157] The absolute value of the irreversible flux loss is small andexcellent heat resistance (heat stability) can be obtained.

[0158] Because of the high magnetic flux density that can be secured bythis invention, it is possible to obtain bonded magnets having excellentmagnetic properties which are equivalent to or higher than those of theconventional bonded magnets manufactured by means of the compactionmolding, even though the bonded magnets are manufactured by means ofinjection molding or extrusion molding. Namely, it is possible to obtainbonded magnets having excellent magnetic properties with maintaining theexcellent moldability and productivity.

[0159] Further, since sufficient magnetic properties can be obtainedwith a relatively small amount of magnetic powder to be contained in abonded magnet composition, not only moldability is improved but alsodimensional precision, mechanical strength, corrosion resistance andheat resistance (heat stability) are further improved, thereby enablingto manufacture bonded magnets having high reliability with ease.

[0160] Since the magnetizability of the bonded magnet according to thisinvention is excellent, it is possible to magnetize a magnet with alower magnetizing field. In particular, multipolar magnetization or thelike can be accomplished easily and reliably, and further a highmagnetic flux density can be obtained.

[0161] Finally, it is to be understood that the present invention is notlimited to Examples described above, and many changes or additions maybe made without departing from the scope of the invention which isdetermined by the following claims. TABLE 1 Average Crystal Grain SizeSample No. Alloy Composition (nm) Comp. Ex. 1(Nd_(0.8)Pr_(0.2))_(8.8)Fe_(bal)Co_(7.5)B_(5.9) 55 This Invention 2(Nd_(0.8)Pr_(0.2))_(8.8)Fe_(bal)Co_(8.0)B_(5.7)Nb_(1.0)Ti_(0.8)Dy_(0.2)32 This Invention 3(Nd_(0.7)Pr_(0.3))_(9.0)Fe_(bal)Co_(5.0)B_(5.7)Cr_(1.0)Mo_(0.2)Hf_(0.3)28 This Invention 4(Nd_(0.5)Pr_(0.5))_(8.9)Fe_(bal)Co_(8.0)B_(5.8)Zr_(0.8)Mn_(0.7)W_(0.5)30 This Invention 5(Nd_(0.4)Pr_(0.6))_(8.6)Fe_(bal)Co_(7.0)B_(5.5)Ti_(0.5)Cr_(0.5)Zr_(0.5)26 This Invention 6(Nd_(0.8)Pr_(0.2))_(8.2)Fe_(bal)Co_(7.0)B_(5.7)Mo_(0.8)W_(0.7)V_(0.5) 35Comp. Ex. 7(Nd_(0.7)Pr_(0.3))_(8.6)Fe_(bal)Co_(5.0)B_(5.8)Dy_(1.0)Mn_(1.0)Cr_(1.5)57

[0162] TABLE 2 Example 1 Irrespective ρ Br H_(cJ) (BH)_(max)(BH)_(max)/ρ² Br/ρ Flux Loss Sample No. (Mg/m³) (T) (kA/m) (kJ/m³)(x10⁻⁹J · m³/g²) (x10⁻⁶T · m³/g) (%) Comp. Ex. 1 5.75 0.71 388 62 1.880.123 −10.0 This Invention 2 5.75 0.79 455 91 2.75 0.137 −2.8 ThisInvention 3 5.76 0.81 524 96 2.90 0.140 −2.6 This Invention 4 5.74 0.80565 94 2.84 0.139 −2.4 This Invention 5 5.76 0.81 541 97 2.92 0.141 −3.0This Invention 6 5.75 0.78 571 88 2.66 0.135 −3.3 Comp. Ex. 7 5.76 0.70468 66 2.00 0.121 −6.6

[0163] TABLE 3 Example 2 Irrespective ρ Br H_(cJ) (BH)_(max)(BH)_(max)/ρ² Br/ρ Flux Loss Sample No. (Mg/m³) (T) (kA/m) (kJ/m³)(x10⁻⁹J · m³/g²) (x10⁻⁶T · m³/g) (%) Comp. Ex. 1 6.10 0.74 387 70 1.880.122 −10.2 This Invention 2 6.12 0.83 452 103 2.74 0.136 −2.9 ThisInvention 3 6.11 0.86 522 108 2.89 0.141 −2.7 This Invention 4 6.10 0.85563 106 2.84 0.139 −2.5 This Invention 5 6.09 0.86 538 108 2.91 0.142−3.1 This Invention 6 6.12 0.82 565 100 2.66 0.134 −3.4 Comp. Ex. 7 6.110.74 463 74 1.99 0.121 −6.7

What is claimed is:
 1. Magnetic powder composed of an alloy compositionrepresented by R_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is atleast one kind of rare-earth element excepting Dy, M is at least onekind of element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, xis 7.1-9.9 at %, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30),and the magnetic powder being constituted from a composite structurehaving a soft magnetic phase and a hard magnetic phase, wherein when themagnetic powder is mixed with a binding resin and then the mixture issubjected to injection molding or extrusion molding to form a bondedmagnet having a density ρ[Mg/m³], a maximum magnetic energy product(BH)_(max)[kJ/m³] of the bonded magnet at room temperature satisfies arelationship represented by a formula of (BH)_(max)/ρ²[×10⁻⁹J·m³/g²]≧2.10, and an intrinsic coercive force H_(CJ) of the bondedmagnet at room temperature is in a range of 400-760 kA/m.
 2. Themagnetic powder as claimed in claim 1, wherein a remanent magnetic fluxdensity Br[T] of the bonded magnet at room temperature satisfies arelationship represented by a formula of Br/ρ[×10⁻⁶ T·m ³ /g]≧0.125. 3.Magnetic powder composed of an alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder being constituted from a composite structure having asoft magnetic phase and a hard magnetic phase, wherein when the magneticpowder is mixed with a binding resin and then the mixture is subjectedto injection molding or extrusion molding to form a bonded magnet havinga density p[Mg/m³], a remanent magnetic flux density Br[T] of the bondedmagnet at room temperature satisfies a relationship represented by aformula of Br/ρ[×10⁻⁶ T·m³/g]≧0.125 and an intrinsic coercive forceH_(CJ) of the bonded magnet at room temperature is in a range of 400-760kA/m.
 4. The magnetic powder as claimed in claim 1, wherein the magneticpowder is obtained by milling a melt spun ribbon.
 5. The magnetic powderas claimed in claim 4, wherein a thickness of the melt spun ribbon is10-40 μm.
 6. The magnetic powder as claimed in claim 4, wherein the meltspun ribbon is obtained by colliding a molten alloy of a magneticmaterial onto a circumferential surface of a cooling roll which isrotating to cool and then solidify it.
 7. The magnetic powder as claimedin claim 6, wherein the cooling roll includes a roll base made of ametal or an alloy and an outer surface layer provided on an outerperipheral portion of the roll base to constitute the circumferentialsurface, in which the outer surface layer of the cooling roll has a heatconductivity lower than a heat conductivity of the roll base.
 8. Themagnetic powder as claimed in claim 7, wherein the outer surface layerof the cooling roll is formed of a ceramic.
 9. The magnetic powder asclaimed in claim 1, wherein said R comprises rare-earth elements mainlycontaining Nd and/or Pr.
 10. The magnetic powder as claimed in claim 1,wherein said R includes Pr and its ratio with respect to a total mass ofsaid R is 5-75%.
 11. The magnetic powder as claimed in claim 1, whereinthe composite structure includes a nanocomposite structure.
 12. Themagnetic powder as claimed in claim 1, wherein the magnetic powder issubjected to a heat treatment at least once during a manufacturingprocess or after its manufacture.
 13. The magnetic powder as claimed inclaim 1, wherein a mean crystal grain size of the magnetic powder is5-50 nm.
 14. The magnetic powder as claimed in claim 1, wherein anaverage particle size of the magnetic powder is 0.5-150 μm.
 15. A methodof manufacturing magnetic powder, in which a melt spun ribbon isobtained by colliding a molten alloy of a magnetic material onto acircumferential surface of a cooling roll which is rotating to cool andthen solidify it, and then thus obtained melt spun ribbon is milled toobtain the magnetic powder, in which the magnetic powder is composed ofan alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting. Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder being constituted from a composite structure having asoft magnetic phase and a hard magnetic phase, wherein when the magneticpowder is mixed with a binding resin and then the mixture is subjectedto injection molding or extrusion molding to form a bonded magnet havinga density ρ[Mg/m³], a maximum magnetic energy product (BH)_(max)[kJ/m³]of the bonded magnet at room temperature satisfies a relationshiprepresented by a formula of (BH)_(max)/ρ²[×10⁻⁹ J·m³/g²]≧2.10, and anintrinsic coercive force H_(CJ) of the bonded magnet at room temperatureis in a range of 400-760 kA/m.
 16. A method of manufacturing magneticpowder, in which a melt spun ribbon is obtained by colliding a moltenalloy of a magnetic material onto a circumferential surface of a coolingroll which is rotating to cool and then solidify it, and then thusobtained melt spun ribbon is milled to obtain the magnetic powder, inwhich the magnetic powder being composed of an alloy compositionrepresented by R_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is atleast one kind of rare-earth element excepting Dy, M is at least onekind of element selected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, xis 7.1-9.9 at %, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30),and the magnetic powder being constituted from a composite structurehaving a soft magnetic phase and a hard magnetic phase, wherein when themagnetic powder is mixed with a binding resin and then the mixture issubjected to injection molding or extrusion molding to form a bondedmagnet having a density ρ[Mg/m³], a remanent magnetic flux density Br[T]of the bonded magnet at room temperature satisfies a relationshiprepresented by a formula of Br/ρ[×10⁻⁶ T·m³/g]≧0.125, and an intrinsiccoercive force H_(CJ) of the bonded magnet at room temperature is in arange of 400-760 kA/m.
 17. A bonded magnet manufactured by mixingmagnetic powder with a binding resin and then subjecting the mixture toinjection molding or extrusion molding, in which the magnetic powder iscomposed of an R-Th-B based alloy having at least one element selectedfrom Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy (where R is at least onekind of rare-earth element excepting Dy, and TM is a transition metalmainly containing Fe), the bonded magnet being characterized in thatwhen a density of the bonded magnet is ρ[Mg/m³], a maximum magneticenergy product (BH)_(max)[kJ/m³] of the bonded magnet at roomtemperature satisfies a relationship represented by a formula of(BH)_(max)/ρ²[×10⁻⁹ J·m³/g²]≧2.10, and an intrinsic coercive forceH_(CJ) of the bonded magnet at room temperature is in a range of 400-760kA/m.
 18. The bonded magnet as claimed in claim 17, wherein a remanentmagnetic flux density Br[T] of the bonded magnet at room temperaturesatisfies a relationship represented by a formula of Br/ρ[×10⁻⁶T·m³/g]≧0.125.
 19. A bonded magnet manufactured by mixing magneticpowder with a binding resin, and then subjecting the mixture toinjection molding or extrusion molding, wherein the magnetic powderbeing composed of an R-TM-B based alloy having at least one elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy (where R is atleast one kind of rare-earth element excepting Dy, and TM is atransition metal mainly containing Fe), the bonded magnet beingcharacterized in that when a density of the bonded magnet is ρ[Mg/m³], aremanent magnetic flux density Br[T] of the bonded magnet at roomtemperature satisfies a relationship represented by a formula ofBr/ρ[×10⁻⁶ T·m³/g]≧0.125, and an intrinsic coercive force H_(CJ) of thebonded magnet at room temperature is in a range of 400-760 kA/m.
 20. Thebonded magnet as claimed in claim 17, wherein the magnetic powder iscomposed of an alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder is constituted from a composite structure having a softmagnetic phase and a hard magnetic phase.
 21. The bonded magnet asclaimed in claim 17, wherein a maximum magnetic energy product(BH)_(max)[kJ/m³] is equal to or greater than 40 kJ/m³.
 22. The bondedmagnet as claimed in claim 16, wherein an absolute value of anirreversible flux loss (initial flux loss) is equal to or less than6.2%.
 23. The magnetic powder as claimed in claim 3, wherein themagnetic powder is obtained by milling a melt spun ribbon.
 24. Themagnetic powder as claimed in claim 24, wherein a thickness of the meltspun ribbon is 10-40 μm.
 25. The magnetic powder as claimed in claim 24,wherein the melt spun ribbon is obtained by colliding a molten alloy ofa magnetic material onto a circumferential surface of a cooling rollwhich is rotating to cool and then solidify it.
 26. The magnetic powderas claimed in claim 25, wherein the cooling roll includes a roll basemade of a metal or an alloy and an outer surface layer provided on anouter peripheral portion of the roll base to constitute thecircumferential surface, in which the outer surface layer of the coolingroll has a heat conductivity lower than a heat conductivity of the rollbase.
 27. The magnetic powder as claimed in claim 26, wherein the outersurface layer of the cooling roll is formed of a ceramic.
 28. Themagnetic powder as claimed in claim 3, wherein said R comprisesrare-earth elements mainly containing Nd and/or Pr.
 29. The magneticpowder as claimed in claim 3, wherein said R includes Pr and its ratiowith respect to a total mass of said R is 5-75%.
 30. The magnetic powderas claimed in claim 3, wherein the composite structure includes ananocomposite structure.
 31. The magnetic powder as claimed in claim 3,wherein the magnetic powder is subjected to a heat treatment at leastonce during a manufacturing process or after its manufacture.
 32. Themagnetic powder as claimed in claim 3, wherein a mean crystal grain sizeof the magnetic powder is 5-50 nm.
 33. The magnetic powder as claimed inclaim 3, wherein an average particle size of the magnetic powder is0.5-150 μm.
 34. The bonded magnet as claimed in claim 19, wherein themagnetic powder is composed of an alloy composition represented byR_(x)(Fe_(1-a)CO_(a))_(100-x-y-z)B_(y)M_(z)(where R is at least one kindof rare-earth element excepting Dy, M is at least one kind of elementselected from Ti, Cr, Nb, V, Mo, Hf, W, Mn, Zr and Dy, x is 7.1-9.9 at%, y is 4.6%-8.0 at %, z is 0.1-3.0 at %, and a is 0-0.30), and themagnetic powder is constituted from a composite structure having a softmagnetic phase and a hard magnetic phase.
 35. The bonded magnet asclaimed in claim 19, wherein a maximum magnetic energy product(BH)_(max)[kJ/m³] is equal to or greater than 40 kJ/m³.
 36. The bondedmagnet as claimed in claim 17, wherein an absolute value of anirreversible flux loss (initial flux loss) is equal to or less than6.2%.